Xylan is a component of plant hemicellulose. Xylan consists of 1,4-glycosidically linked .beta.-D-xylose. Usually xylans have side chains containing xylose and other pentoses, hexoses and uronic acids.
In the paper production process the bleaching of pulp is an important step. Schematically the steps used in the pulp treatment in paper and pulp industry is performed as follows:
Pulp is treated at pH 10-12 at 80.degree. C. to remove most of the lignin in the so-called oxygen delignifying step. The remaining pulp contains 2-5% of lignin. This lignin gives the pulp the brown color. Subsequently, the pulp is bleached in a multistage bleaching process. In this bleaching chemicals such as chlorine, chlorine dioxide, hydrogenperoxide and/or ozone are used to obtain a pulp for high quality paper.
Chlorine and chlorine-containing chemicals are often used to remove lignin, which is responsible for the brownish color of the pulp. Use of the indicated chemicals leads to the formation of dioxin and other chlorinated organic compounds. These compounds form a threat to the environment and there is a growing tendency to omit the use of chemicals giving rise to similar waste products.
This has prompted a tendency to develop chlorine-free processes; total chlorine free (TCF) and elementary chlorine-free (ECF). In these processes hydrogen peroxide or ozone is used for bleaching.
It has been found that the introduction of an enzymatic step in the paper and pulp preparation process has several advantages.
Xylanases have been found to be very useful in the paper and pulp processing. Xylanases have been reported to increase the extractability of lignins from the pulp. Xylanases are mostly used after the oxygen delignifying step.
Xylanases cleave the hemicellulose chain linking the lignin to the cellulose chain. After xylanase treatment the lignin is more easily removed in the subsequent steps.
Therefore the use of xylanases leads to a reduction of the consumption of active chlorine in prebleaching of 25-30%. This reduction of chlorine does not afflict the quality parameters of the resulting paper (Viikari et al. 1986. Proc. of the third Int. Conf. Biotechnology in Pulp and Paper Ind., Stockholm, p.67-69 and Bajpai and Bajpai. 1992. Process Biochemistry. 27: 319-325).
The xylanase treatment also reduces the need for other chemicals in the bleaching process.
The use of xylanases from fungal sources in bleaching of kraft pulp has been reported. The pH and temperature optima of these enzymes are: pH=3-5 and T=30-50.degree. C. These values are not ideal for the use in the bleaching process where the prevailing conditions are pH.gtoreq.9 and temperature .gtoreq.70.degree. C.
Xylanases from bacterial origin, with higher pH and/or temperature optima have also been reported for use in the bleaching process. Some of these are the following:
Bacillus pumilus (pH=7-9, T=40.degree. C., Nissen et al., 1992. Progress in Biotechnology 7: 325-337), Dictyoglomus thermophilum (pH=6-8, T=70.degree. C., European patent application EP 0 511 933), B.stearothermophilus T-6 (pH=9.0, T=65.degree. C., Shoham, Y. et al. (1992) Biodegradation 3, 207-18), B.stearothermophilus (pH=9, T=50.degree. C., WO 91/18976) and Thermoanaerobacter ethanolicus (68.degree. C., Deblois and Wiegel. 1992. Progress in Biotechnology 7: 487-490).
Even though most of the above cited xylanases show activity at pH.gtoreq.9 and temperature &gt;70.degree. C., their effectiveness under industrial application conditions (i.e. during the bleaching of pulp), in terms of e.g. increased brightness of the pulp is only limited and can vary significantly (see e.g. WO 91/18976, highest increase in pulp brightness at pH 9 and 50.degree. C. is only 0.5% ISO brightness).